Re: Proposal: Add conspicuously missing Functor instances for tuples

El 19 ene 2016, a las 10:24, Alois Cochard escribió:

+1

Agree for consistency, I can also see those instances as being useful in some specific context, even if I agree with Andreas that in general they should be discouraged (especially for newcomers).

Can you give us an example where using e.g. the Functor instance for a 5-tuple would be the correct/best design decision? Tom

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On 19 January 2016 at 09:20, Herbert Valerio Riedel wrote: On 2016-01-18 at 21:10:07 +0100, David Feuer wrote:

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For some reason I really can't imagine, it seems the only tuple type with a Functor instance is (,) a. I was astonished to find that

fmap (+1) (1,2,3)

doesn't work. Since this is *useful*, and there is *only one way to do it*, I propose we add the following:

instance Functor ((,,) a b) where fmap f (a,b,c) = (a,b,f c) instance Functor ((,,,) a b c) where fmap f (a,b,c,d) = (a,b,c,f d) etc.

As stated elsewhere in this thread already, there is the issue about consistency. Here's a relevant section from the Haskell 2010 report[1]:

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6.1.4 Tuples

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However, every Haskell implementation must support tuples up to size 15, together with the instances for Eq, Ord, Bounded, Read, and Show.

IMO, we either have no `Functor` instances for tuples at all, or we have them for all tuples up to size 15. The current situations of having them defined only for 2-tuples is inconsistent.

Cheers, hvr

[1]: https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1210006... _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- Λ\ois http://twitter.com/aloiscochard http://github.com/aloiscochard _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries